1. Field of the Invention
The invention relates generally to a tapered roller bearing, and more specifically to a tapered roller bearing incorporated in, for example, a final reduction gear unit of an automobile and lubricated with oil stored in the final reduction gear unit.
2. Description of the Related Art
FIG. 12 illustrates a conventional tapered roller bearing. The tapered roller bearing includes an outer ring 72, an inner ring 82, a plurality of tapered rollers 74, and a cage 87. The outer ring 72 has an outer raceway surface 71 formed on its inner peripheral face. The inner ring 82 has an inner raceway surface 81 formed on its outer peripheral face. The tapered rollers 74 are rollably disposed between the outer raceway surface 71 and the inner raceway surface 81. The cage 87 holds the tapered rollers 74 at equal intervals in the circumferential direction of the tapered roller bearing.
Tapered roller bearings are widely used because they are compact, able to bear large radial loads and large axial loads, and usable under high-speed rotation. However, toque loss of the tapered roller bearings is larger than that of ball bearings. Therefore, reduction of toque is a matter to be addressed from the viewpoint of energy saving.
The tapered roller bearing is configured such that, under the pumping action resulting from the rotation of the tapered roller bearing, the oil introduced into the tapered roller bearing through a clearance between a small-diameter-side end portion 88 of the cage 87 and an inner ring outer periphery 85 and a clearance between the small-diameter-side end portion 88 of the cage 87 and an outer ring inner periphery 75 flows through the tapered roller bearing. The stirring resistance of the oil largely contributes to an increase in the rotary torque of the tapered roller bearing. Japanese Patent Application Publication No. 2005-69421 (JP 2005-69421 A) describes a configuration in which a small-diameter-side portion of the cage 87 is bent radially inward to reduce a clearance between the small-diameter-side portion of the cage 87 and the outer periphery of the inner ring 82 (hereinafter, referred to as “radially-inner clearance”) and to form a labyrinth, in order to appropriately adjust the flow of oil in the tapered roller bearing, thereby reducing the rotary torque. Japanese Patent Application Publication No. 2008-202785 (JP 2008-202785 A) describes a configuration in which means for adjusting a clearance between the small-diameter-side end portion 88 of the cage 87 and the inner periphery of the outer ring 72 (hereinafter, referred to as “radially-outer clearance”) is provided in order to limit the flow of oil through the radially-outer clearance.
The tapered roller bearing described in JP 2008-202785 A is used to support a counter drive gear of a vehicle automatic transmission. In this support structure, the oil that has flowed through the bearing is discharged into a closed space where no oil path for returning the oil into an oil reservoir is formed. When the closed space is full of the oil, part of the oil introduced into the bearing flows out of the bearing through the radially-outer clearance.
On the other hand, when the tapered roller bearing is applied to a final reduction gear unit, a circulating oil path is formed in a space into which the oil is discharged, and thus the oil flows from the radially-outer clearance in such a direction that the oil flows into the bearing. It is deemed that the amount of oil flowing through the bearing may be restricted to a certain extent by reducing the radially-outer clearance.
However, it is necessary to ensure a small radially-outer clearance to restrict the amount of oil flowing in the bearing. Generally, the cage is guided by rolling elements, and the position of the cage varies by an amount corresponding to a clearance between the rolling element and the wall face of a cage pocket. Therefore, in order to ensure a small radially-outer clearance while preventing the cage from coming into contact with the outer ring during rotation of the tapered roller bearing, it is necessary to manufacture the cage with minimized dimensional variations. Thus, strict dimensional control that involves a large amount of time and effort is required.